1. Field of the Invention
This invention relates to ceramics and more specifically to the field of dental porcelains used to fabricate artificial teeth.
2. Description of the Prior Art
Prior ceramic=glass composite materials used in the construction of artificial teeth are characterized by a microstructure containing a crystalline ceramic in a vitreous matrix. Thus, it is known that aluminous ceramic materials contain 40 to 50% alumina in a borosilicate glass matrix. Upon firing, the resulting ceramic body has a microstructure consisting of crystalline alumina dispersed in a glass matrix. The fired alumina composite body has a diametral tensile strength of around 9,000 psi and a coefficient of thermal expansion of 8.times.10.sup.-6 /.degree.C. In British Pat. No. 1.105,111, magnesium aluminate spinel-glass composites have also been used in the construction of artificial teeth. The raw material for this ceramic contains a mixture of alumina and a smaller amount of magnesia present in a ratio suitable for the formation of magnesium aluminate spinel and unreacted alumina. The fired ceramic body microstructure consists of magnesium aluminate and alumina crystalline phases in an alkaline earth silicate glass matrix. In U.S. Pat. No. 4,265,669, a fired magnesium aluminate-glass composite has a tensile strength of 9,000 psi and a coefficient of thermal expansion below 8.times.10.sup. -6 /.degree.C. Both of these prior art ceramic core materials are unsuitable for bonding to high expansion dental porcelains due to the danger of cracking produced by thermal stresses because the coefficient of thermal expansion difference tolerance for bonding ceramics to porcelain is less than 100%. Since the coefficient of thermal expansion of high expansion feldspar porcelains range from range from 12 to 14.times.10.sup.-6 /.degree.C., the prior art ceramics described above are unuitable for bonding to feldspar porcelains. The prior art ceramics can only be used for bonding to low expansion dental porcelains. This is a serious disadvantage since dental laboratories have the high expansion porcelains available for use in the construction of porcelain fused to metal dental crown and bridgework.